Extraintestinal pathogenic Escherichia coli (ExPEC) are major pathogens in mostly the very young, aged or immunocompromised human population resulting in tens of thousands of deaths each year and billions of dollars in healthcare costs. For example, the predominant ExPEC subtype, E. coli O18:K1, expresses O18 somatic and K1 polysialic acid capsule antigens as two major virulence factors responsible for making these strains the leading causes of neonatal bacterial meningitis. Using microbial genetics and innovative methods of carbohydrate analysis, we defined the complete pathways for sialic acid transport, synthesis, polymerization, catabolism and modification of both monomeric and polysialic acids. The modification mechanism was linked to a novel bacterial virus (CUS-3) carrying the phase-variable acetylase gene catalyzing stochastic O-acetylation of the sialic acid exocyclic chain. We hypothesize that understanding the in vivo functions of these bacterial antigens will facilitate approaches targeting them for new therapeutic development. We have designed three specific aims to test this hypothesis: 1) Determine the contribution of monomeric sialic acid O- acetylation to overall polysialic acid modification by genetically altering the acetyltransferase, NeuD, and acetylesterase, NeuA* (NeuA-star). This aim focuses on the enzymes involved in the reciprocal addition (NeuD) or subtraction (NeuA*) of O- acetyl esters to or from monomeric sialic acids prior to their incorporation into polysialic acid. 2) Determine the dynamics of capsular polysialic acid modification in vivo using an innovative flow cytometric technique and microscopic methods to distinguish between acetylated and unacetylated phases in different host compartments. 3) Determine the molecular basis for coupling polysaccharide synthesis to export by (i) constructing in frame deletions of all region 1 export genes, (ii) determining whether 3-deoxy-D-manno- octulosonate is required for biosynthesis, (iii) identifying the polymerase domain(s) interacting with the accessory protein, KpsC, (iv) using Quantum-dot technology and K1- specific phage to determine the site of capsule export, and (v) determining the chemical structure of the initiation complex.